Two basic types of electrosensitive gel materials exist. The first type includes certain gelled copolymers which, when placed in aqueous solution, undergo reversible contraction or expansion in response to very small changes in pH. Typically, the pH changes are induced by the application of electrical potential differences to the solution, thus producing an electrically controllable response. The response actually takes the form of a change in volume of the expandable copolymer resulting from uptake of the solution at high pH levels, or release of the solution at low pH levels. Thus, the response is said to be linear and homogenous in the sense that, for example, a long cylindrical piece of gel would undergo the same amount of relative percentage change in length and diameter. Percentage changes of more than 400% have been observed. Expandable copolymer gels of this type are described in U.S. Pat. No. 5,100,933 (Tanaka, et al); in U.S. Pat. No. 5,250,167 (Adolf, et al); and in U.S. Pat. No. 5,389,222 (Shahinpoor, et al), the disclosures of which are incorporated by reference herein.
Expandable copolymer gels of these types may comprise three dimensional networks consisting of polyacrylic acid that can be obtained by heating a foil of polyacrylic acid containing a polyvalent alcohol such as glycerol or polyvinyl alcohol. The resulting three dimensional network is insoluble in water, but swells in response to high pH and contracts in response to low pH. Electric fields in the range of a few volts per centimeter suffice to stimulate that response.
Thus, for example, U.S. Pat. No. 5,250,167 (Adolf, et al) discloses a variety of mechanisms based on encapsulated polyelectrolyte polymeric gels in aqueous electrolytic solutions, which undergo reversible expansion and contraction in response to electric fields in the range of a few volts per centimeter as a result of changing the pH of the solution. Adolf discloses that filaments of copolymer gel may be proposed for most applications. The specific machines he describes and depicts in FIGS. 1 through 6 are quite simple and are constrained by the need to immerse the copolymer fibers in an aqueous electrolyte solution, which must not be allowed to leak away. Fundamentally, his machines rely upon simple linear contraction. A critical point is that, while both an anode and a cathode are required, the fibers ordinarily should be connected to only one of the two electrodes, leaving a gap 26 shown in FIGS. 1 and 2. According to Adolf, if the gap is omitted and opposite ends of the copolymer gel fibers are connected to electrodes of opposite polarity, the result would be expansion near one electrode and contraction near the other, with little or no net change in length.
Fundamentally, Adolf discloses only simple push-pull mechanisms (FIGS. 1-2), bending mechanisms (FIGS. 3-4), and an oscillator (FIG. 6). (Adolf's FIG. 5 is a push-pull mechanism in which the electrodes that change pH of the solution are physically separated from the copolymer gel; the gel responds to pumping of high or low pH solution into its container.)
Other investigators have suggested bendable structures as shown in FIG. 3 of U.S. Pat. No. 5,389,222 (Shahinpoor, et al) in which one side of a sheet of expandable copolymer gel is made to expand, while the other side is made to contract in response to an electrical field applied across the section of the sheet. (This, of course, takes advantage of the effect of eliminating the gap mentioned in FIGS. 1-2 of the Adolf '167 patent). Shahinpoor also discloses a sphincter-like device (FIG. 4) which closes in response to the application of an electrical field. He also mentions a spring-loaded device (FIG. 5) in which an oscillating rotary motion is produced by contraction of a gel element.
The second type of electrosensitive gel involves both a different electrochemical mechanism and different mechanical results. Such gels are variously termed electrorheological gels or ER gels or fluids, that exhibit a phenomenon called the Winslow effect. These ER gel or fluid materials typically comprise a dielectric fluid in which is dispersed a plurality of microscopic electrorheologically sensitive particles. Application of an electrical field to such a composite material alters the pattern of electrical charge distribution on the surface of the electrorheological particles, causing them to be attracted to each other and to become aligned in a regular fashion, effectively forming chains of microscopic fibers between the electrodes. The electrorheological particles may include silica, starch, carboxy-modified polyacrylamides, and similar materials which will function only in the presence of some water. Other materials such as organic semiconductors, including silicone ionomers, are said to be capable of functioning without water. See, for example, U.S. Pat. No. 4,772,407 (Carlson); U.S. Pat. No. 5,032,307 (Carlson); U.S. Pat. No. 5,252,249 (Kurachi, et al); U.S. Pat. No. 5,252,250 (Endo, et al) ; and U.S. Pat. No. 5,412,006 (Fisher, et al), the disclosures of which are incorporated by reference herein.
In either case, the salient characteristic of ER gels is that the application of a voltage difference results in a macroscopic change from liquid-like behavior to essentially solid behavior. That is, the ER fluids or gels change from behaving as Newtonian fluids, which deform continuously and without limit in response to the application of any stress (force) at all, to Bingham plastic fluids, which will not deform at all until some threshold level of yield stress (force) is applied. The storage modulus G' and the loss modulus G" also change dramatically in response to application of voltage gradients to these materials. (These moduli relate to the ability of the material to damp energy.) Electrical current flows are said to be low, and response times are of the order of milliseconds.
ER gels are used in automotive transmissions, clutches, vibration dampeners, and brakes. One investigator suggests using such gels as base materials for the ink used in ink jet printers. See U.S. Pat. No. 5,326,489 (Asako, et al). In addition, U.S. Pat. No. 5,213,713 (Reitz) proposes a variety of simple shapes comprising various beams, angles, and the like which include one or more portions made of ER gel. By temporarily applying an electrical potential to the electrorheological solid portion of such items, their shape can be changed by mechanically bending the item and then removing the electrical field, whereupon the electrorheological solid portion "freezes" into the new shape. Notably, however, most ER gels exhibit behavior opposite to that described by Reitz: that is, usually the application of an electrical field to an ER fluid or gel results in solidification, not liquefaction.
Among the drawbacks of the prior art in the area of expandable copolymer gels are the comparatively slow response times and the need for immersion in water. And although ER gels have rapid response times, they are not suited for direct creation of motion.